1. Field of the Invention
The present invention relates to a solid-state image pickup device and a manufacturing method therefor and, more specifically, to a solid-state image pickup device operating in an interlace mode with a light shielding film provided, and a manufacturing method therefor.
2. Description of the Related Art
A solid-state image pickup device of a conventional type operating in an interlace mode is structured by a flat N-type silicon substrate 111 formed thereon with a photoelectric conversion region 112 as shown in FIGS. 16A and 16B. Specifically, FIG. 16A is the layout view, and FIG. 16B shows the schematic cross-sectional view. The photoelectric conversion region 112 has the upper layer of a hole accumulation layer 113 that is a P+-type layer, and the lower layer of an N-type layer 114. The N-type layer 114 of the photoelectric conversion region 112 is formed deep into the silicon substrate 111. This is because the hole accumulation layer 113 is so formed as to appear on the surface of the silicon substrate 111. One side of the photoelectric conversion region 112 is formed with a vertical register 116 via a reading gate 115. This vertical register 116 has the upper layer of an N-type layer 117, and beyond thereto, a P+-type layer 118 is located. For partitioning a pixel region, a pixel separation region 119 is formed. Via this pixel separation region 119, the other side of the photoelectric conversion region 112 is formed with another vertical register 116 for partitioning from the pixel locating adjacent thereto. This vertical register 116 has the upper layer of an N-type layer, which is formed as closer as possible to the surface of the silicon substrate 111 to carry the charge as much as possible. On the vertical register 116 and the reading gate 115, an electrode 122 is formed with an insulation film 121 locating therebetween. This electrode 122 serves as an electrode for charge reading and transferring. In this example, shown is an electrode in a two-layer structure. A light shielding film 133 is formed with an opening 132 locating on the photoelectric conversion region 112, via an inter-layer insulation film 131.
In a general manner for charge reading from the photoelectric conversion region 112 to the vertical register 116, voltage is applied to the electrode 122 serving as an electrode for charge reading and transferring so that the vertical register 116 and the reading gate 115 are both changed in potential. The voltage is continuously applied to the electrode 122 until the reading gate 115 becomes lower in potential than the N-type layer 114 of the photoelectrical conversion region 112.
The issue here is that changing the potential of the reading gate 115 to be lower than that of the N-type layer 114 of the photoelectrical conversion region 112 requires considerably high voltage. This is because, in the conventional structure, the N-type layer 114 of the photoelectrical conversion region 112 is formed deeper in the silicon substrate 111 compared with the N-type layer 117 of the vertical register 116. The potential of the reading gate 115 is modulated by lateral diffusion due to the heat of the hole accumulation layer 113. This moves the maximum potential of the reading gate 115 serving as a charge reading path deep down into the silicon substrate 111. As a result, the potential at the position shows less change with respect to the reading voltage, problematically increasing the reading voltage to a greater degree.
Furthermore, there has been a demand for pixels smaller in size in consideration of the expected trend of higher resolution. With the concern for maintaining the pixel properties, however, it has been difficult to reduce the height difference of electrodes and others to a greater degree than currently achieved. Thus, reducing the pixels in size may reduce the light-gathering capability.
A solid-state image pickup device with a groove is found in Patent Document 1. Therein, for the purpose of reducing the reading voltage and expanding the control margin of the reading voltage, a photodiode and a vertical transfer section are both arranged in an array. The surface of a substrate has a groove for use as a channel opposing to a reading electrode and a transfer electrode. These electrodes are those provided for charge reading and transferring from the photodiodes to the vertical transfer sections.
As such, to meet the demand of smaller-sized solid-state image pickup devices and higher resolution, the vertical transfer section is required to handle the more amount of charge. This is the reason why, in Patent Document 1, the substrate is formed with the groove as the expected solution. In detail, to increase the amount of charge for the vertical transfer section to handle, the side parts of the groove are used also as the vertical transfer section so that the effective area for charge transfer is increased. Furthermore, in consideration of another demand for lower power consumption, there needs to reduce the reading voltage from the photo diode section to the vertical transfer section.
[Patent Document 1] JP-A-11-97666 (pages 3 to 4, and FIG. 1)
The issue here is that such a structure has the following problems as forming a groove on the surface of a substrate for use as a channel opposing to a charge reading electrode and a charge transfer electrode. That is, end parts of a polysilicon electrode operating for charge reading and transferring are not located in the groove but merely on the substrate. Thus, reducing the height difference of the polysilicon electrode is difficult. The polysilicon electrode is not placed directly above the reading gate. The potential of the reading gate is thus modulated by lateral diffusion due to the heat of a hole accumulation layer. This accordingly moves the maximum potential of the reading gate serving as a charge reading path deep down into the silicon substrate. As a result, the potential at the position shows less change with respect to the reading voltage, failing to solve the problem of increasing reading voltage. Here, even when a light-shielding film is formed to cover the polysilicon electrode may be a possibility, the resulting film simply covers the polysilicon electrode on the surface of the substrate, failing to fully achieve the expected light-shielding effect, and reducing the smear characteristics. Moreover, although reducing the reading section in width may be an effective option for voltage reduction, this easily causes blooming. In consideration as such, it has been expected to increase the amount of charge to be handled by the vertical transfer section simultaneously with reducing the reading voltage.